1. Field of the Invention
The present invention relates to an air conditioning system that utilize refrigerants and a compressor, and particularly to an air conditioning system that can prevent compressor driving chamber from reaching an abnormally high-pressure state.
2. Description of the Related Art
A known air conditioning system is disclosed in Japanese Patent Application No. 7-19630 and includes a compressor 1, a cooling circuit 51, a heating circuit 52 and a controller 83, as shown in FIG. 1. The cooling circuit 51 includes a condenser 55, a first expansion valve 57 and a heat exchanger 59 provided on a passage connecting a discharge port D to a suction port S of the compressor 1. High pressure refrigerant discharged from the discharge port D of the compressor 1 is drawn through the above respective devices and back to the compressor 1.
The heating circuit 52 includes a bypass passage 52a extending from the discharge port D of the compressor 1 to a second expansion valve 63 provided within the bypass passage 52a and to the heat exchanger 59. The high pressure refrigerant discharged from the compressor 1 is not directed to the condenser 55, but rather is drawn by the compressor 1 through the second expansion valve 63 and the heat exchanger 59. Such a heating circuit 52 is generally known as a hot gas bypass heater.
The operation of the cooling circuit 51 and the heating circuit 52 is changeably selected by opening and closing selector valves 53a and 53b, which opening and closing operations are performed by the controller 83.
Because the refrigerant discharge pressure is higher when the heating circuit 52 is used than when the cooling circuit 51 is used, the air conditioning system must operate in a high pressure state when the heating circuit 52 is utilized. An abnormally high-pressure state may be created if the output discharge capacity of the compressor 1 temporarily increases during the operation of the heating circuit 52. A refrigerant releasing passage 91 having a pressure relief valve 93 is provided in order to release excess pressure from the air conditioning system, if an abnormally high pressure state is reached. The refrigerant releasing passage 91 is connected to the heating circuit 52 and the cooling circuit 51 and the pressure relief valve 93 can be opened to release the refrigerant from the heating circuit 52 into the cooling circuit 51 when the refrigerant discharge pressure abnormally increases during the operation of the heating circuit 52.
Because the cooling circuit 51 and the heating circuit 52 are alternatively selected by the selector valves 53a and 53b, the refrigerant is released into the cooling circuit 51 which is not used during operation of the heating circuit 52, thereby preventing the discharge pressure at the heating circuit 52 from increasing abnormally.
However, because the refrigerant in the heating circuit 52 is released into the cooling circuit 51 whenever the discharge pressure abnormally increases, the amount of refrigerant in the heating circuit 52 is reduced and heating performance may be reduced. Moreover, because the high pressure refrigerant is wastefully released from the heating circuit, energy efficiency is reduced.
Another variable displacement compressor is disclosed in Japanese Patent Application No. 10-47242. Although this compressor is not explicitly shown in the drawings, a connecting passage having a capacity control valve is provided between a discharge port and a driving chamber (also known as the crank case) in a housing such that refrigerant is released from the discharge port into the driving chamber when the capacity control valve is opened. Moreover, the driving chamber and a suction port communicate with each other such that the refrigerant in the driving chamber is sent to the suction port. When the discharge pressure is excessively high or suction pressure is excessively low in the compressor, the capacity control valve is opened to release the refrigerant from the discharge port into the driving chamber, thereby increasing the pressure in the driving chamber. As a result, the output discharge capacity of the compressor decreases and thus, the discharge pressure decreases and the suction pressure increases. In order to maintain the output discharge capacity decreasing effect by maintaining pressure in the driving chamber, a throttle can be provided between the driving chamber and the suction port.
If the variable displacement compressor disclosed in Japanese Patent Application No. 10-47242 is employed in an air conditioning system having the hot gas bypass heater circuit disclosed in Japanese Patent Application No. 7-19630, a problem can occur due to a combination of a characteristic of the variable displacement compressor and a characteristic of the hot gas bypass heater.
As described above, because the high-pressure refrigerant is released from the discharge port into the driving chamber in order to decrease the output discharge capacity, the pressure in the driving chamber increases significantly. Although the driving chamber and the suction port communicate with each other, it takes time to send the refrigerant from the driving chamber to the suction port and the pressure in the driving chamber is maintained in a high-pressure state during that time. Especially in the above case in which a throttle is provided between the driving chamber and the suction port, because a long time is required to send the refrigerant from the driving chamber to the suction port, the high-pressure state in the driving chamber continues for a long time.
Therefore, in the known variable displacement compressor, the driving chamber seal is designed based on the assumption that the interior of the driving chamber will be subjected to high-pressure states. Especially during operation of the heating circuit (hot gas bypais heater) which uses the refrigerant at higher pressure as compared with the cooling circuit, the pressure of the refrigerant released from the discharge port into the driving chamber may increase over an expected steady-state value in some cases and the interior of the driving chamber may reach an excessively high-pressure state in such cases. When the excessively high-pressure state is caused in this manner, airtight seal of the driving chamber is degraded and the durability of the compressor may be also degraded.